A symbol of Westley and Buttercup's love for one another would be the first thing to think of. Traditional symbols of love include red roses, hearts, and rings (like a wedding ring). As the prevailing theme of the book is how true love conquers all, one of the artifacts or objects should have to do with this concept of love.
Something that symbolized social status and money could be another artifact, such as gold coins or some sort of symbol of high social status. As evidenced by Westley starting out as a "Farm Boy" and Buttercup bosses him around, Buttercup is of a social station slightly higher than Westley. Many characters in the book only care about money, like Humperdink and Vizzini.
Prince Humperdink only cares for Buttercup's beauty and does not care about her as a person. Buttercup is used for her beauty. An object that could symbolize this is something that is gilded or gold or silver plated -- it is painted or plated with a precious metal, but underneath it has no real worth. Often jewelry or utensils or things like that will be made of a cheaper metal with a thin layer of precious metal on top.